1. Field of the Invention
The present invention relates to a method of doping silicon bodies through indiffusion of boron and, more particularly, to an open diffusion method of doping silicon bodies.
2. Description of the Prior Art
For diffusing boron into silicon, capsule diffusion methods are known as well as methods wherein silicon bodies are exposed to a gas which flows through a reaction tube and contains a boron source. With capsule diffusion methods, highly homogeneous and reproducible results can be obtained. Capsule diffusion methods, however, have disadvantages in the amount and complexity of material and apparatus required. For example, each batch run requires an expensive quartz capsule in addition to a high vacuum pump and its associated annealing furnaces. Furthermore, capsule diffusion methods involve a relatively high amount of manual operation such that it may not readily be incorporated in an automatic production line. Capsule diffusion methods are also not readily useable in those diffusion processes where, after the diffusion, an oxide is required to be grown for subsequent masking purposes.
Boron diffusions by means of a gas stream which contains a boron source and which flows past silicon bodies (hereinafter called "open diffusion") are much less complex insofar as apparatus and material is concerned. In addition, manual intervention is required relatively infrequently and thus, this type of diffusion is quite suitable for incorporation into an automatic production line. In typically employed open diffusion processes, a boron glass is produced on the surface of the silicon body which is to be doped with boron by heating the silicon bodies to a high temperature and exposing same to a gas stream containing oxidized boron. The oxidized boron is introduced into the gas stream by directing latter past boron-nitride wafers coated with B.sub.2 O.sub.3 and heated to a high temperature. Alternatively, oxygen in the gas stream may be made to react with a material that is fluid (at room temperature) or gaseous and which contains boron.
An open diffusion process using a boron source comprising boron-nitride preferably coated with B.sub.2 O.sub.3 is described, for example, in German Offenlegungsschrift 23 16 520. Open diffusion processes where boronbromide (BBr.sub.3), fluid at room temperature, is used as a boron source are described, for example, in U.S. Pat. No. 3,676,231 to B. P. Medvecky et al, and in the articles entitled "The Influence of the Reaction Kinetics of O.sub.2 and Source Flow Rates on the Uniformity of Boron and Arsenic Diffusions" in Solid State Electronics, 1971, Vol. 14, pp. 281ff, and "The Influence of Reaction Kinetics Between BBr.sub.3 and O.sub.2 on the Uniformity of Base Diffusion" in Proceedings of the IEEE, Vol. 57, No. 9, September 1969, pp. 1507ff. The description of an open boron diffusion process using as boron source, B.sub.2 H.sub.6 gaseous at room temperature, is described, for example, in the article entitled "Ellipsometric Investigation of Boron-Rich Layers on Silicon" by K. M. Busen et al in the Journal of the Electrochemical Society, Vol. 115, March 1968, pp. 291ff.
If the BBr.sub.3 :O.sub.2 ratio is not too low in the open diffusion processes, a silicon-rich boron phase (SiB.sub.6) is obtained under the boron glass directly placed onto the silicon, as described in the above-cited article by K. M. Busen et al. As described there, the forming of the SiB.sub.6 phase is advantageous for a well-controlled boron diffusion. This favorable effect, however, appears only when the SiB.sub.6 phase is removed prior to the boron drive-in in the second heating process. Busen et al recommend for the removal of the SiB.sub.6 phase either an etchant which also etches silicon, or using a two-step process where in the first step the phase is treated for ten minutes in boiling concentrated nitric acid and subsequently for 30 seconds is diluted HF. The use of silicon etchants is not advisable for silicon wafers upon which integrated circuits are to be applied and, thus, the two-step process may have to be applied several times to make sure that the SiB.sub.6 phase is completely removed, as described by Busen et al.
It is clear that because of the lack of reliability of the etching process, the two-step method of Busen et al is not suitable for a manufacturing application. In their article entitled "Glass Source B Diffusion in Si and SiO.sub.2 " published in Journal of the Electrochemical Society, Vol. 118, February 1971, pp. 293ff, D. M. Brown and P. R. Kennicott therefore suggest a boron diffusion (with reference to the article by Busen et al) where the forming of the SiB.sub.6 phase is avoided. This of course also excludes the attendant advantages of the SiB.sub.6 phase. In the method described in German Offenlegungsschrift 23 16 520, the advantages mentioned by Busen et al are utilized in that the SiB.sub.6 phase is converted, after its formation in a high temperature oxidation step, into an easily soluable oxide which is removed prior to the second heating process. In spite of these process steps, the method described in the Offenlegungsschrift has generally not been found to present homogeneous and reproducible results in the fabrication of low-doping diffusion areas (surface resistance: &gt;300 .OMEGA./.quadrature., penetration depth: approximately 1 .mu.m ) within very close tolerances over the silicon body and over an entire batch.
With the increasing trend towards closely packed, highly integrated, circuits the results obtained by means such as described in the above-cited Offenlegungsschrift are no longer fully satisfactory. In the production of highly doped areas, the intensive oxygen treatment after the boron oxide coating can have a very deleterious effect in many applications. Although it can be seen from the above-mentioned article that Busen et al attempted tests where, prior to the drive-in, only the boron glass and not the silicon-rich phase is removed, it is clear from the results obtained there that Busen et al have concluded that for obtaining favorable diffusion results the SiB.sub.6 phase has to be removed prior to drive-in.